One of the goals of electricity metering is to accurately measure the use or consumption of electrical energy resources. With such measurements, the cost of generating and delivering electricity may be allocated among consumers in relatively logical manner. Another goal of electricity metering is help identify electrical energy generation and delivery needs. For example, cumulative electricity consumption measurements for a service area can help determine the appropriate sizing of transformers and other equipment.
Electricity metering often involves the measurement of consumed power or energy in the form of watts or watt-hours. To this end, meters include voltage sensors and current sensors that detect, respectively, the voltage and current delivered to the load. In most cases, the purpose of the voltage sensor is to provide a measurement signal that represents a scaled version of the voltage waveform delivered to the load. Similarly, a current sensor provides a measurement signal that represents a scaled version of the current waveform delivered to the load. In many cases, other circuitry within the meter calculates one or more energy consumption values (kW-hr, VA-hr, etc.) by digitizing the voltage and current measurement signals and performing various known calculations using the digitized measurement signals.
The current measurement in a utility meter can be challenging because a high accuracy is required over a large range, and common current sensor technologies can be susceptible to various sources of error. Present metering technologies involve current transformers, or CTs. Existing CT designs are prone to saturation and may distort causing error especially in a DC magnetic field or with a half wave rectified load. To compensate for such errors, additional circuitry is often required, which increases costs. Such additional circuit can include an additional compensation winding that is used to provide a compensation signal to the CT to improve the accuracy of the signal generated by the main secondary winding. For example, U.S. Pat. No. 4,255,704 shows a compensation method used in utility metering which involves additional compensation windings and many specialized electronic circuits with precision parts. This requirement makes the manufacture of the specialized metering CT complex and expensive.
One alternative to the ordinary CT is a current sensor that is based on a Rogowski coil, which is a coil wrapped around a non-magnetic core. A Rogowski coil, unlike an ordinary CT, is relatively immune to the negative effects of DC currents, and is immune to constant magnetic fields, such as those associated with permanent magnets. While a current sensor based on Rogowski coil has the advantage of immunity to DC current and permanent magnets, the Rogowski coil has the disadvantage of being particularly sensitive to AC signal coupling.
In particular, FIG. 8 shows a traditional prior art Rogowski coil current sensor topology for use in electricity metering. The current to be measured is represented as the source Ip, and the primary coil, which may suitably be a single turn conductor passing through an opening in the Rogowski coil core, is represented as L1. The winding L2 of the Rogowski coil is, according to the prior art, coupled to the non-inventing input of an amplifier U1, while the inverting input of the amplifier U1 is coupled to a reference voltage. The output OUT is operably coupled to an A/D converter or the like to provide current measurement signals. A capacitor C is coupled in the feedback path from the output OUT to the non-inverting output.
In operation, the Rogowski coil's output is a low level voltage ({tilde over (V)}RC) that is directly proportional to the derivative of the primary side input current (Ĩp). The output voltage is then integrated to recover the phase and amplitude of the primary current. Due to the low voltage levels produced by a Rogowski coil L2, the device is inherently sensitive to near electro-magnetic fields. In particular, nearby AC voltages (shown as VAC) may couple capacitively through Ce. This capacitively coupled VAC induces an error current that will result in an offset error out the integrator output.
The error current (Ĩe) due to the capacitive coupling of VAC through Ce will be divided through the secondary of the Rogowski coil (L2) and the resistor R of the integrator connected to the floating ground at inverting terminal of U1. Moreover, the Rogowski coil secondary has a series resistance due the resistance of the wire. Due to the presence of the error current, as the primary current decreases, the signal to noise ratio of the integrator decreases, causing the ratio error to increase. The resistance of the Rogowski coil must be smaller than R by at least 2 to 3 orders of magnitude in order to achieve acceptable accuracy over normal meter temperature ranges. Even then, AC coupling can still be an issue. To address these errors, shielding may be used to block the VAC. Alternatively, software corrections may be employed to correct the offset error. However, such solutions are non-ideal because of complexity and cost, among other things.
A single ended integrator configuration could be improved by increasing the gauge of the wire. Decreasing the resistance by ten will decrease the amplitude of the error voltage by ten. However the size and cost of the Rogowski coil is undesirably increased in such a solution.
There is a need, therefore, for a current sensor arrangement that favorably improves upon one or more of shortcomings of existing transformers, for example, by providing sufficient accuracy under various circumstances while reducing production cost.